Another potential way to stop plants from overrunning eachother is by adding humus. Humus makes the ground nutrient-rich so plants will grow better and potentially faster, so also spreading seeds earlier and faster and thus also increasing the chances of all the different plant's existence in a single forest. A.k.a a forest existing out of different types of trees and small plants and because of that also habitable for multiple different animals (so I can finally have my huge forest with mice and owls).

It could very well be that this mechanic is already in the game since humus is literally dirt and that I'm overthinking things... but still!
I also read the dev was busy on structures. So maybe players could willingly make wildfires to increase the amount of humus in the soil? (Or to just watch stuff burn, of course...)

I'm still not too sure how to implement the biome division (see Sand topics).

And yes, I'm currently very busy working on structure.
And, maybe we an also use structure to divide biome.
And special magical fence that prevent plant from spreading.
But I have no idea how I can code this... it can slow the game also.

Minerals are a thing. Plant variety in nature is often ensured by having a variety of nutrients - each plant has different demands, so a single plant cannot claim an area forever, it will run out of that specific nutrient, so other plants have to take over. Once the other resources deplete, the old ones might have came back, so the cycle starts again.